That "gut" feeling that moves one person to trust another is governed by a brain hormone most noted for its role in the mother- child bond.
This finding, reported by the Society for Neuroscience, is the closest anyone has come to unraveling the brain's role in guiding the universal social phenomenon of trust.
Paul Zak, director of the Center for Neuroeconomic Studies at Claremont Graduate University in Southern California, designed a study to get at the question of how two human beings decide to trust one another. An economist and a neuroscientist, he chose money as his research tool.
Forty college students signed on for a study on decision-making. Each was given $10 and ushered to a computer. Each was teamed with an anonymous partner, who sat at one of the other 39 computer stations in the room. They would never know the identity of their partner.
Half the group was instructed to give any amount of their $10 to their partner, a stranger somewhere in the large laboratory. In making that decision on a specific dollar amount, the receiver then would get three times that amount, plus their original $10. There was no guarantee that the receivers would get anything.
Then the second person was asked to decide how much money he or she wanted to give back to the first decision-maker.
Afterward, all of the volunteers gave blood, and it was screened for the blood hormone oxytocin and nine other hormones.
Oxytocin is found in both men and women. It was chosen for the trust study because it is known to play a role in maternal bonding and social recognition.
The more money the second decision-maker gave to a partner, the higher his or her blood levels of oxytocin, Zak found.
In the study, 85 percent of the first decision-makers decided to send money to their partner. And 90 percent of the second decision- makers sent some money back.
"No one could really tell us why they made a particular decision," Zak explained. "But the more oxytocin available in the blood, the more they sent."
He said that the hormone is active in the amygdala, an area of the brain that governs a variety of emotional states. It also is the home of the classic fear response that protects humans in dangerous conditions, including threat by another individual.
"It's as if the brain is telling you the right thing to do socially," Zak said.
On average, those who made the first move walked out of the room with $12. The second movers made $18.
"We have an internal guiding system that, because we are social beings, we have to learn how to care and respond socially," Zak said. "This hormonal response affects their conscious decision process."
Women who were ovulating at the time of the study had high oxytocin levels but gave back far less money to their partners compared with their nonovulating counterparts.
Zak suspects that when women are ovulating, the possibility of becoming pregnant makes them less trusting, thus protecting them from making poor decisions during this time in the cycle.
Zak and his colleagues now are repeating the study in populations of people with social or emotional problems.
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Last updated: 11/27/2003 - 10:22 AM